In optoelectronic devices of the kind mentioned above, the light wave guides lie in the wafer along the electrodes in the interaction region. An incoming light wave is acted on by the effective refractive index in the light wave guides being changed with the aid of an electric field through the light wave guides. This field is generated by an electric signal which is connected to the electrodes. For high modulation frequency, when the electric signal is a microwave signal, there are problems with phase separation between the light wave and the microwave, which limits the modulator bandwidth. The light wave has greater velocity than the microwave in the interaction region so that the two waves come into counterphase at the end of the interaction distance and the modulation is counteracted. Phase separation can be reduced by making the interaction distance shorter, although this requires a correspondingly higher voltage in the modulating microwave signal. It is complicated and expensive to generate a high frequency microwave signal with high voltage, and the problem with phase separation has been attacked in different ways, e.g. as is apparent from the Swedish patent application No. 8305572-2. According to this application, one of the modulator electrodes is divided into two halves at half the interaction distance, these halves being mutually connected via a condenser. The electrode halves are each connected to a d.c. voltage at a suitable level and polarity, whereby the phase separation is counteracted. In the British patent application No. 2109580 meanderformed electrodes have been proposed for simulating a velocity of the microwave which is in agreement with the velocity of the light wave. The electrodes are complicated to manufacture and result in a modulator with small bandwidth.
In the above-mentioned apparatus, the propagation of the microwaves has been acted on for suitably modulating a light wave. It is known from another technical field, i.e. the transmissio of microwaves, to utilize tubular wave guides for the microwave. A coaxial cable is connected to the cylindrical surface on the tubular wave guide and the central conductor of the cable projects into the wave guide. The tubular wave guide has a closed end, and the coaxial cable is connected at a distance corresponding to a quarter of a wave length of the microwave from this end. The projecting central conductor functions as an antenna transmitting the microwave so that a travelling microwave occurs in the wave guide and a standing wave occurs at the end of the wave guide. A more detailed description of this apparatus is to be found, e.g. in R. E. Colling, "Foundations for Microwave Engineering", Mc Graw Hill Book Company, New York 1966, pp 183-187. However, the apparatus has not found any application in modulating a light wave.